1. Field of the Invention
The present invention relates to a differential signal receiver, and particularly to a differential signal receiver used to receive a small-amplitude differential clock signal and a differential data signal.
2. Description of Related Art
Thin and low-power consumption flat panel displays are mainstream for PCs (personal computers) and television monitors. Liquid crystal display panels, which are a main format of the flat panel display, are rapidly advancing to be developed, and the numbers of display devices and effective pixels are desired to increase. Liquid display panels display a screen by converting digital signal data being input into analog voltage to apply it to liquid crystal devices. With an increasing number of pixels and effective pixels, the input digital signal data needs to be transferred faster. Accordingly the data transfer is carried out by data transfer standards using a small-amplitude differential signal such as RSDS (Reduced Swing Differential Signaling) and miniLVDS (Low Voltage Differential Signaling). Therefore, in a display driver for processing data, a receiver is required for converting an amplitude of a small signal differential signal data being input into an internal logic voltage.
A block diagram of a general liquid crystal panel is shown in FIG. 5. The liquid crystal display panel includes a display control apparatus 101, a power supply circuit 102, a source driver 103, a gate driver 104, and a TFT-LCD (Thin Film Transistor-Liquid Crystal Display) 105. The display control apparatus 101 supplies display data being input and a control signal to the source driver 103 and the gate driver 104. The power supply circuit 102 is housed in the display control apparatus 101 to supply a reference voltage to the source driver 103 and the gate driver.
The source driver 103 controls pixel lines in columns of the TFT-LCD according to the display data and the control signal. The gate driver 104 controls pixel lines in rows of the TFT-LCD according to the control signal. The TFT-LCD 105 is a liquid crystal panel with pixels arranged in matrix to display an image.
An internal block diagram of the source driver 103 that receives the display data is shown in FIG. 6. The display data is data indicating color information, for example. The display data is transferred by a small amplitude differential signal. The source driver 103 receives the display data with the receiver 110 and converts a differential signal into a single-end signal. The single-end signal and the control signal are converted from digital to analog signals in an internal logical circuit 111.
Further, a voltage and an amplitude for the display data fluctuates due to a fluctuation in production tolerance, reflection, and noise in a transmission. This causes the signal output from the receiver to fluctuate delay time and deteriorate a duty ratio indicating a ratio of high and low levels in a waveform. A receiver to improve this fluctuation is disclosed in Japanese Unexamined Patent Application Publication No. 2003-198265 (first conventional technique).
A block diagram of the receiver according to the first conventional technique is shown in FIG. 7. The receiver of the first conventional technique level shifts a direct-current level of a differential signal received via input terminals A and B in a direct-current level converter 121, amplifies it by a first stage amplifier 122 (gain G1), a second stage amplifier 123 (gain G2), and a third stage amplifier 124 (gain G3), converts the differential signal to a single-end signal in the third stage amplifier, and then outputs the single-end signal from an output circuit. A detailed circuit diagram of the receiver is shown in FIG. 8.
As shown in FIG. 8, a direct-current level converter 121 includes NMOS transistors QN1 and QN2 connected to input terminals A and B respectively. Drains of the NMOS transistors QN1 and QN2 are connected to a power supply potential VDD, and current sources I1 and I2 are connected between sources and ground potential GND. The sources of the NMOS transistor QN1 and QN2 output internal output signals int_OUTa and int_OUTb corresponding to differential signals INa and INb input respectively input from the input terminals A and B. Direct-current levels of the internal output signals int_OUTa and int_OUTb are values of a direct-current level Vcm of the differential signals INa and INb subtracting a threshold voltage Vgs. A signal waveform of an input signal is shown in FIG. 9A, and a signal waveform of an output signal from the direct-current level converter 121 is shown in FIG. 9B. As shown in FIGS. 9A and 9B, the differential signals INa and INb are signals with a center at the direct-current level Vcm, a high level voltage at VH, a low level voltage at VL, where an amplitude VIN indicated with a voltage difference between VH and VL. Further, the internal output signals int_OUTa and int_OUTb that are output from the direct-current level converter 121 have amplitudes of VIN with a center at the current level Vcm−Vgs.
The first stage amplifier 122 includes NMOS transistors QN3 to QN6. Sources of the NMOS transistors QN3 and QN4 are connected in common, and connected to the ground potential GND. Further the NMOS transistors QN5 and QN6 are diode connected, and are connected between drains of the NMOS transistors QN3 and QN4, and the power supply potential VDD. A gate of the NMOS transistor QN3 is connected to the source of the NMOS transistor QN1, and receives one output signal output from the direct-current level converter 121. A gate of the NMOS transistor QN4 is connected to the source of the NMOS transistor QN2, and receives another output signal output from the direct-current level converter 121. NMOS transistors QN3 and QN4 switch to be conductive or nonconductive according to a signal input to the gates, amplify the signals input from their drains to the gates, and output the signals.
The second stage amplifier 123 includes an amplifier 123a for amplifying an output from the NMOS transistor QN4 and an amplifier 123b for amplifying an output from the NMOS transistor QN3. The second stage amplifier 123 amplifies each signal by the gain G2. The third stage amplifier 124 includes an amplifier 124b, amplifies the signals output from the amplifiers 123a and 123b, and converts the signals into single-end signals to output the signals. An output circuit 125 controls whether to output the signals output the signals output from the amplifier 124 to an output terminal OUT.
The receiver according to the first conventional technique is able to reduce delay time of the signals in the receiver by amplifying the signals by the plural stages of amplifiers, even if the amplitude of the differential signals fluctuate.
However in the receiver of the first conventional technique, if the direct-current level Vcm of the input signal shifts to a side of the power supply voltage VDD or the amplitude VIN of the input signal enlarges, an amount of the current flowing to the NMOS transistor QN3 or QN4 increases, thereby increasing the threshold voltage Vgs of the NMOS transistor QN5 and QN6 and also reducing the delay time of the receiver.
On the other hand if the direct-current level Vcm of the input signal shifts to a side of the ground potential GND or the amplitude VIN of the input signal reduces, an amount of the current flowing to the NMOS transistor QN3 or QN4 decreases, thereby decreasing the threshold voltage Vgs of the NMOS transistor QN5 and QN6 and also increasing the delay time of the receiver.
Further, with a lower limit of an input range of the direct-current level Vcm of the input signal indicated by Vlimit, in order for the receiver to operate, it must be Vlimit1>Vgs(QN1)+Vgs(QN3)−(VIN/2). In case Vgs=1V and VIN=200 mV, Vlimit>(1+1−(0.2/2))=1.9V. Thus the receiver is not able to operate with the direct-current level Vcm having the input signal lower or equal to 1.9V.
Accordingly in the receiver according to the first conventional technique, it has now been discovered that if the amplitude of the input signal or the direct-current level fluctuates, the delay time of the signal also fluctuates. Further, there is another problem that if the direct-current level of the input signal falls below the lower limit of the input range of the direct-current level of the input signal, the receiver does not operate.